1. Field of the Invention
This invention relates to a gallium nitride rectifying device using a gallium nitride based semiconductor.
2. Description of the Related Art
A gallium nitride (GaN) has a wider bandgap width and a higher breakdown field intensity than those of silicon (Si), and has an excellent heat resistance, thus it is suitable for a semiconductor material directed at a high-power electronic device such as a rectifying device and a transistor. For example, in a pn junction diode that is a typical rectifying device. GaN is overwhelmingly excellent in properties in comparison with not only Si that is a conventional material but also silicon carbide that is recently developed extensively.
FIG. 4 is a graph showing a relationship in theoretical limitation between a breakdown voltage of the pn junction diode and an on resistance of the device (for example, refer to a nonpatent literature 1 (J. W. Johnson et al., IEEE Trans. Electron Device, ED-49 (2002), 32)). Further, the theoretical limitation of FIG. 4 shows an on resistance in a simple structure to which microfabrication such as a trench structure is not applied. There is a trade-off relationship between a high breakdown voltage and a low on resistance that are needed for a high-power device. In FIG. 4, comparing between the same breakdown voltages, GaN has an on resistance of not more than 0.1% of Si, thus it is expected that loss can be extremely reduced.
In recent years, it is reported that a device having characteristics superior to Si by using GaN is going to be realized (for example, refer to a nonpatent literature 2 (N. Ikeda et al., ISPSD 2008, p. 289)).
In addition, in Si, a structure enhanced in a performance by that microfabrication such as a super junction (SJ) structure is applied thereto is developed. It is considered that this structure is also effectively used in the other material systems, thus a similar effect on performance enhancement can be expected. Consequently, microfabrication itself does not close the performance gap provided by the material characteristics. Namely, it is quite possible to extend a performance of a GaN device beyond the performance limitation shown in FIG. 4 by structural contrivance.
JP-A-2007-234907 (patent literature 1) discloses that a concentration of acceptor impurity included for the purpose of heightening the breakdown voltage (reverse voltage resistance) or for the other purpose is controlled to less than 1×1019 cm−3.
JP-T-2005-530334 (patent literature 2) discloses that a high breakdown voltage can be realized by controlling a dislocation density in crystal to not more than 5×106 cm−2.